High-frequency tube structure



July 22, 1952 s. F. VARIAN I HIGH-FREQUENCY TUBE STRUCTURE Original Filed Jan. 29, 1942 3 Sheets-Sheet l BY ayflm ATTORNEY July 22, 1952 s. F. VARIAN 2,604,605

HIGH-FREQUENCY TUBE STRUCTURE Original Filed Jan. 29, 1942 5 Sheets-Sheet 5 l lg. 8

ATTORNEY Patented July 22, 1952 HIGH-FREQUENCY TUBE STRUCTURE 1 I Sigurd F. Varian, Garden City, N. Y., assignor to The Sperry Corporation, a corporation of Delaware Original application January 29, 1942, Serial No.

428,682. Divided and this application December 14, 1946, Serial No. 716,209 I '25 Claims.

This invention relates, generally, .to ultra-highfrequency vacuum tube structurey' and, more specifically to ultra-high-frequency devices of the electron beam type such as disclosed in Patents No. 2,245,627, entitled Stabilization of Frequency," issued June 17, 1941, in the name of Russell H. Varian, No. 2,250,511, entitled Oscil lator Stabilization System, issued July'29, 1941, in the names of Russell H. Varian and William W. Hansen and No. 2,259,690, entitled High Frequency Radio Apparatus, issued October 21, 1941, in the names of John R. Woodyard, William w Hansen and Russell H. Varian. The present application is a division of application Serial No. 428,682 filed January 29, 1942,' issued as Patent No. 2,414,785 on January 21, 1947. In the aforementioned disclosures, there is shown an ultra-high-frequency device consisting of- ;an indirectly heated I cathode which projects an electron beam through grids which form portions of the opposite walls of a hollow-cavity conducting resonator. After the electron beam leaves the exit grid of the;resonator, it is refiected back through the resonator by means of a reflector electrode operating at or very near to'the potential of the cathode. If a high frequency alternating electric field exists ,between the resonator grids due to the presence of a corresponding high frequency alternating electromagnetic field in the resonator, the electron beam suffers recurrent velocity changes by the action of saidalternating electric field during its initial passagethrough the resonator.- As the elecs tron 'beam passes through the exit grid of the resonator and onward,-the effect is to commence to produce recurrent grouping of the electron beam. .The spacing. between the resonator exit grid and the reflector electrode, and the voltage on said electrode, are so arranged that the electrons are reflected back into the resonator at a time when the recurrent velocity changes have resulted in recurrent grouping of the electron beam and at a time such that these electron groups returncnergy to the alternating electric field appearing between the resonator grids, thus maintaining the oscillating electromagnetic field in-. side of the resonator. Direct, current energy is thus introduced into the device by means of the electron beam, and may be removed'as ultrahigh-frequency energy from the hollow resonator by means of coupling loops and associated concentric line elements, as more fullydescribed in the aforementioned patents. t

One object of the presentinvention is: the provision of improved tuning means for high frequency tube structures of the type employing a single resonator such as disclosed in the abovementioned patents, together with suitable reso-, nator, cathode, and reflector electrode shapes and structure for operating such devices at wavelengths on the order of ten centimeters or less.

A further object of the invention is to provide novel types of indirectly heated cathode structures suitable for use in the small spacings involved in. short wavelength hollow-cavity resonant devices. I e 1.

Another object is to provide temperature com; pensation of the frequency of such a device by suitable choice of materials and design. 7

Still another object is to provide such an ultra high-frequency device with a resonator constructed so that desirable close spacing of reso:

nator grids, cathode, and deflector plate may be achieved. I

A further object is to provide mechanicaltuning means whereby the grids of a single hollow cavity resonator electron beam velocity modulating device of the aforementioned type may be moved parallel to each other, thus maintaining parallelism of the cathode and reflector plate structures associated therewith.

Fig. 1 is a cross sectional elevational view, of one form of tube structure according to one aspect of the present invention. 0 7 v t Fig.2 is a fragmentary cross-section of a detail of Fig. 1, showingthe cathode construction.

Fig. 3 is a perspective view of a modification of the structure shown in Fig.2. r r Fig. 4 is an alternate form of the structure of Fig. 2.

Fig. 5 is a partial cross-sectional elevational view of an alternate form of the structure shown inFig. l.v Y

Fig. 6 is a fragmentary perspectiveview of a detail of Fig. 5,. Fig. 7 is a fragmentary cross-sectional elevational view of an alternate form of the cathode structure shown in Fig. 5. I i

Fig. 8 is a partial. cross-sectional elevational view of an alternate form of Figs. 1 and'5.

Fig. 9 is a modification of a detail of Fig. 8.

in Fig. lthere is shown a single resonator electron beamrvelocity-modulation tube structure-of the type disclosed in the aforementioned p'at-:

ents. Electrons from emitter'l may be pro-' jected in a beam of round cross-section through entrance grid 8 of ahollow, conducting resonator 9. Resonator 9, as shown, has a-r'een-' trant portion 10, a flexible end wall ll, an outer cylindrical wall 12, anda rigid fiat end wall l3 opposite to wall I 5. End wall I3 of resonator 9 is apertured and carries a grid structure I4 concentric with and close to grid 8. Grids 8 and [4 may, as shown in Fig. 5 of parent application 428,682, consist of an annular member provided with alternate long and short radial conducting bars inserted in rings which in turn are inserted in reentrant portion I0 and end wall [3. The

elements of grids 8 and M maybe made of zir- Y I conium or other metals which emit relatively few secondary electrons. Spaced concentric to grid l4 and close behind it is=reflector electrode I I8 which may be dish-shaped as shown. Re-

flector I8 is supported by conductor I9 which isbutt-sealed near the periphery of flange 66.

Suitable cathodes for use in a tube of'the type r shown in Fig. l inwhich the cathode is mountpasses through glass end bell 20, which is; in r turn supported by and sealed to end wall Iiiot resonator 9 in the conventional manner.

In operation, the electron beam emitted from cathode l is projected through entrance grid 8,"

into resonator 9. In this resonator the beam sufiers recurrentvelocity changes or-is velocitymodulated by means of an oscillating electric field existingbetween" grids 8 and M. The beam travels out through exit grid land is reflected back through grid I4 by a voltage applied to re'- flector I8 very near that of the cathode voltage, the reentry of the beam into resonator 9 taking place after recurrent bunching or density modulation of the beam has taken place, said grouped electron .beam then giving up energy to'theelecf tric field between grids I4 and 8, thus'maintaining-oscillations in resonator 9. I j

Resonator 9 may be provided with coaxial line terminal posts'or elements2-I and'22, emerging from resonator end wall I3; which may be similar in construction to that shown'in Fig. 401' the above-mentioned parent application. V c

For tuning resonator '53, flanges '29 and=3fl are connected respectively to end plate I3 (and hence to grid I i) and to reentrantportionlfl (and he'nceto grid 8); A plurality of screws 31 are threaded into flange 29 in which are seated struts 6| whose other ends are seated in flange 30; In this way adjustment of screws 3.! serves tjoi'ad just the spacing of grids tunes the resonator. p; The tub structure shown in Fig. 1 has alow frequency shift with thermal changes and other advantages to be furthere'xplained. Frequency shift may be brought about from "changes in the spacing between grids 8 and I4 due to thermal expansion or contraction. As will be apparent from a study of Fig. 1, such spacing change originates largely from two sources: expansionor contraction of the cylindrical portion Ill bearing entrance grid 8, and expansion or contraction of struts 6| and associated rough tuning screws 3?. In Fig. 1, the reflex device has low thermal coefiicient'of frequency. Flanges 29an'd 3fl'maybe made of steel, or of other metal of low thermal conductivity, in order to avoid excessive temperature-rise of the tuning mechanism thereto "at tached. 4 Reentrantportion In of resonator 9 continues beyond wall I l and integrally flares out to flanged portion parallel to flanges '29, 39,'tubeIl being of one of the well known high nickel content alloys suitable for butt-sealing to certain glasses" and having verylow thermal eoefficient of expansion and preferably having all of itss'urfaces inside of resonator 9 plated With copper or other highly conducting material. Flange 38 is attached directly to the extended portion of cy1in-' der- I0, between resonator 9 and flared portion 60. Struts (H are of :Invar or other material of 8 and Id and heater resistance wire.

ed adjacent to entrance grid 8 and inside of reentrant tube portion I0 are shown in Figs. 2, 3,

and. Becauseofsmall spaces available in reflexos'cillatorsand amplifiers operating in the i region below ten centimeter wavelengths, more conventional types of cathode structures as shown 'in parent Fig. 1 may not be desired. A structure which may he more readily adapted tosuchsmallspacings is shown in-Fig. 2-.- Thermalconductive rod '14, as of tungsten-or other material having a relatively'high melting point, is positioned coaxially with the axis of the elec'- tron beam to be p'roduceda A -f-rustum 75 of aright'ci'rcular cone has a small diameter end, of diameter'substantially equal to that of conductor "I4, welded or otherwise secured to the upper end of conductor '14} 'Frustum I5 may; be of nickel o'r other heat-conducting material, and may have its large diameter surface coated '-with an oxide electron-emission material, as at I6. The cylindrical 'sides of rod l' i are coated with A1203 or; other material of low electrical 'conductivity at high temperature; as at- 81, andaround said coating may be wound a coil 11 of Theheater assembly'is surrounded by heat shield "18, which may be a tube of nickel, and which positions: the heater assembly by means to'bedescri bed. Heatshield I8 may-have insrtedin its lowerend a double wallbottom, such as formed by-walls "I9, -80. Heatshield'IS extends above emitter su'rface 1B 'a -distan'ce sufficient to 'give the resultant electronibe'am a slightly focused quality.-

Conducting rod 14 has its lower end terminated in a cone 8|, the apex'of the'conebeing positioned by a dimple in'heat shield wall As a point contact only is-thus made 'between rod 74 andend .wall 80, very little heat transfer is allowed. I The other degree of free'dom'of motion of the heaterstru'cture inside of shield 18 is removed by making theupper'sharp edge of the frustum i5 fit into an annular con'cave groove 82 near the top or heat shield 18 end of substa'n tially' arcuate cross-section." Suchconstruction results in a line contact, 50 that-very small heat exchange-is allowed between the fr'u'stum l5 and heat shield 18. One'end of'heater coil 71 'may' be spot welded to heat shield 18, as at 84, while the other" end is insula'ted from heat-shield-M by passing out through holes in 'bottom walls 19, 8B. Leads 83 attached to shie1d'18 pass through the glass press of the vacuum envelope of the tube and provide a rigid support for the cathode structure, and, together with-lead85, supply the necessary heater 'Voltages.

It :may' 'be desirable to us'e'a larger-wattage heater coil, in-"place of coil Tl of- Fig; Z'in which case the structure-of Fig. 4 may be used.. The. conducting member "is now extended wellpa'st flange 6D" and is of diameter more nearly'th'at of the inner diameter of heat shield 78'. As it passes beyond flange 60, heat shield 18"is enlarged to -fit'a cylinder 85 Which may"a1so beo f' are inserted.

nickel and which is closed at its lower end .by end wall 88. The portion of conductor 14 covered with A1203 and surrounded by heater coil 89 may be equal in diameter to-the remainder. of rod 14, or may be of somewhat greater'diamete-n, as desired, allowing coil 89 to be more'easily assembled and to be of greater power capacity. It isto be understood that heater coils 11 and 89 may be of the coiled coil variety if desired,- or may be any type of non-inductive winding. 3

If frustum 15 and heat shield 18 are both of nickel, welding may result due to the usual high operating temperature of such a cathode at the line contact between frustum 15 and annular groove 82, resulting inincreased heat transfer between the cathode and heat shield. The structure of Fig. 3 may be used to remedy this undesired result. The larger diameter of the frustum 15 is made smaller than the inner diameter of heat shield 18 so that three wires 90, 90, 90" of molybdenum .or other suitable material may be inserted between the frustum and the heat shield. The wires 90, 90, 90" may be held in position by pressure only or may be bent into slots 9|, 9I', 9I" cut into the upper face ofheat shield 18, the material of the heat shield. adjacent to the slots then being staked over to hold wires 90, 90, 90" firmly in proper placement. The heater assembly may be held in positionby a slint tension on lead wires 95, 85', of heater coil 89.

For much shorter output wavelengths, modiflcation of the shape of the resonator may be useful in providing sufficient space for the cathode and the reflector electrode, as is shown in Fig.5. Here resonator 92 is made by turning out of a round conducting disc a cavity having the shape of a frustum of a right circular cone, thereby forming an outer wall 93 and an upper apertured wall 94 which carries exit grid 95 of the resonator 92 (where such grid is used). A flexible conducting diaphragm 96 and reentrant apertured portion 91, which is also a frustum of a right-circular cone, form the remainder of the resonator boundary. Reentrant portion 91 carries entrance grid '98 (when used) placed concentric with and close to exit grid 95.

Grids 95 and 98 may, as shown in Fig. 6, consist of radial conducting long and short wires 99 and I00. These grid wires may be inserted in wall -94 and reentrant member 91 by milling radial slots in the upper surfaces of 94 and 91, placing the grid wires in the slots in the correct positions, and exerting hydraulic pressure to those faces great enough to cause the surface metal to flow over the wires,'thus staking them in position, or by any other method which will leave relatively smooth surfaces after the grids Outside of the resonator, reentrant .portion 91 may be extended as a tube -IOI and is provided with a flange I02 cooperating witha flange I03 attached-to resonator body 93, said flangesbeing used with any desired mechanical tuning mechanis'm, such as that of parent Fig. l. The emitter surface of cathode I04 is placed close to entrance grid 98, and well inside of tube Il. Cathode I04 may be similar to that shown in Figs. 2 to 4 or of the type shown in Fig. '1 yet to be described,

and may be surrounded by a tubular focusing shield I05, which extends past, emitter I04 towardgrid 98, and which may be supplied with avoltage relative to cathode I04 sufficient togive slight convergence to the resultant electron beam. Focusing shield I05 issupported and supplied with proper voltage by leads I06, which pass through glass press I01.'. Reflector plate I08 isshown as a flat disc with a short cylindrical wall or flange projecting toward exit grid 95 from the .peripheryof said disc.

Coupled with resonator 92 by, coupling loop I09 is a coaxial .line through which ultra-high-frequency energy may be removed, consisting of small diameter portion H0, tapered portion III, and larger diameter portion H2 containing a glas seal H3 to provide continuity of the vac-- uum envelope. All three of the sections have the same inner and outer conductor diameter ratios at any given cross-section in order to avoidimpedance discontinuities in the line. Tapered section I-II is provided so that portion H0 may be of small enough diameter to flt the small resonator 92 used. It is evident that any desired number ofv such coupling loops and concentric line terminal posts may be provided with any of the structures herein shown.

The modified form-of cathode shown in Fig. '1 is adapted for use in structures such as that shown in Fig. 5. This cathode consists of a metal tube II4 which acts as a heat shield, projecting past emitter surface H5 to afiord a slight focusing of the electron beam thereby produced, and

containing heater coil II 6. Emitter surface H5 is made very thin compared to the thickness of tube I I4, and may be oxide coated. The cathode assembly is held rigidly by a truss system of wires II1 arranged in V-formations and supported upon the outer conductor H8 of a coaxial line having an inner conductor H9 that is sealed inside of tube I I8 by glass to metal seal I20. The tubular outer conductor may be used as one lead to heater coil H6, while inner conductor H9 is used as the other lead to heater I I6, the accelerating voltage being applied between H8 and IN.

It is found that for reflex devices of the above type, and especially for shorter wavelength tubes, it becomes desirable to maintain the resonator grids always parallel so that the associated cathode and reflector electrode structures remain in alignment. One form of structure for producing this result is shown in Fig. 8. In this device, resonator I is formed by a cylindrical outer wall I35, a rigid end wall I48 carrying a central exit grid I31,"a flexible end wall I32, and a reentrant pole I35 carrying entrance grid I36. Cathode I39 and reflector I are aligned with grids I36, I31 on opposite sides thereof.

Cylindrical outer wall I35 and reentrant tube I36 of the resonator I34 are considerably extended, and a secondflexible diaphragm I38 is used to close the open end of tube I35. Pressure exerted by a tuning lever 43 of a mechanical tuning mechanism which may be similar to that of parent Fig. 1 forces tube I35 to always move parallel to tube I36, thus maintaining grids I36 and I31 parallel. This method of construction is seen to eliminate some of the complexity of the tuning device, and is seen to also be useful with two resonator electron beam devices.

To obtain perfect alignment of grids I36, I31, cathode I39, and reflector electrode I40, it may be desirable to provide adjustment means for positioning the cathode and reflector. Resonator I34 has its upper wall I48 extended to form flange I4I. Mounted above wall I48 is tube I42 which carries in its open end apertured flexible diaphragm I43. Mounted in the central aperture of diaphragm I43 is tube I44 which is sealed to glass insulator I45 which supports conductor I49 on which reflector electrode I40 is mounted.

7. Tube I44 alsocarries flange I46, parallel to and spaced from flange MI. Three screws I41 are mounted at 120 intervals to rotate freely in holes near the periphery of upper flange I46.

The other ends of screws I41 are threaded into flange I 4I and thereby afiord means of adjustment to secure parallelism between exit grid I31 and reflector I 30 and-the proper spacing of. these elements- Cathode I39, which is mounted on concentric line I50 which projects through glass insulator II in the mannerdescribed in connection with Fig. '7, and which extends up into the reentrant tube I36, may besimilarly adjustably'by the'action of three spaced screws I52 cooperating with flanges I53 and I54 to cause motion of flexible diaphragm I55.

It may be desired to provide the flexible diaphragm means I43, I55 for preliminary adjustment of cathode and reflector spacings at the factory, also providing means to prevent possibility of misalignment at a future date. As seen in Fig. 9, in an illustration showing the method as used on the reflector end of the tube, flanges MI and I46 and cooperating'screws I47 of Fig. 8 are dispensed with. Tube I42 'ismade to extend well above flexible diaphragm I43, as at I57. After alignment of reflector plate I and grid. I31, solder or other low melting alloy"I56 may be poured into the volume defined by tube I51, diaphragm I43, andtube I44. After the alloy cools, the jig or other clamping device used in determining the optimum position of the reflector plate may be removed, the alloy thus securing the position of reflector I40. procedure may be used in permanently positioning cathode I39 of Fig. 8.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departure from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense. I clairn: \f a 1. A high frequency tube structure comprising a cavity resonator havingone wall thereof provided with a reentrant portion, said reentrant portionbeing apertured and being made of a high nickel content alloy having a low thermal coeflicient of. expansion, said-resonator having an apertured wall portion opposite said reentrant portion, 'both of said apertures being aligned and forming an electron-permeable gap, 9. flexible diaphragm adjacent said reentrant portion, tuning means for varying the. axial extent of said gap, said tuning means consisting in part of flang connected to said reentrant portion and.

said apertured wall portion respectively, and; a thermionic cathode structure contained at least partially within said reentrant portion for supplying electrons for' passage through said gap. 2. A high frequency tube structure as defined in claim 1 wherein said cathode comprises a cen tral rod having an enlargedhead providing an electron emitting area, a heating coil surround ing said rod, and heat shielding means surrounding said heating'coil, at least a portion of said heat shielding means projecting into there entrant portion of said resonator wall.

3. A cathode structure comprising. a central heat conducting rod having a pointed end portion and an enlarged endportion, a heatingcoil sur- A similar I ends of said rod, the

8. rounding said rod intermediate its end portions, and a heat shield surrounding said coil and projecting beyond said endportions, the portion of said heat'shield projecting beyond the enlarged endportionpf the rod constituting a focusing electrode:- f A I 1 4. A-cathode structure as definedin claim 3 wherein the portion-ofsaid heat shield projecting beyond the pointed end of said rod is provided with an end wall abutting said pointed end portion, the enlarged end portion also having a bearingupon said heat shield thereby aflixing said rod in place within said shield.

5. A cathode structure "for high frequency tubes comprising an indirectly heated thermal conductor having an enlarged end portion, said end portion having an emitting area, a'heat shield surrounding'said thermal conductor in spaced relation therefrom, said heat shield having a" portion thereof projecting forwardly of said emitting area, said forwardly projecting portion serving as a focusing shield for electrons leaving said emitting area. w

6. High frequency tube apparatus comprising anindirectly heated cathode element having an emitting area, a heat shield surrounding said emittingelement in spaced relation therefrom and having a portion thereof projecting forwardly of said emitting area, said forwardly projecting portion constituting a focusing shield for electrons leaving said emitting area, a hollow resonator having an apertured reentrant wall receiving a portion of said cathode structure, said resonator having an apertured wall opposite its reentrant wall, and a reflector electrode positioned exteriorly of saidjopposite apertured wall, said reflector electrode'being formed with a peripheral focusing portion.

' 7. A cathode structure for ultra-high-frequency tubes comprising an inner heating coil, a cy1indric'al relatively thick tube surrounding said heating coil and constituting the exterior wall of said cathode structure and also a heat shield, and a relatively thin emitter platecontained within said tube near one end thereof and adjacent to said heating coil tobe heated by the latter, a portion of saidtube projecting forward- 1y of said emitter plate and serving as a focusing electrode.

8. A high frequency tube structure comprising a cavity resonator having one wall thereof provided with a reentrant portion, said reentrant portion being apertured, and a cathode structure contained at least partially within said reentrant portion for supplying electrons forpassag through said resonator, said cathode structure comprising a central rod having an enlarged head providing an electron emitting area, a heating coil surrounding said rod, and heat shielding means surrounding said heating coil, at least a portion of said heat shielding means projecting into the reentrant portion of said resonator wall. "9. A cathode structure comprising a central heat conducting rod having an enlarged end provided with a thermionic emissive surface, a'heating coil surrounding said rod and a heat shield surroundingsaid coil and extending beyond the v portion of said heat shield pro ecting beyond the enlarged end portion of said rod serving as a focusing electrode.

10. High frequency apparatus comprising a cathode element having an emitting area, a heat shield surrounding said cathode element 'in spaced'relation therefrom and having a portion thereof projecting forwardly. of said emitting quency tubes comprising an inner heating coil,

a relatively thick tube surrounding said heating coil and serving as the electron wall of said cathode structure and also as'a heat shield, and an emitter element having low. heat capacity contained within said tube near one end thereof and adjacent to said heating .coilto be heated by the'latter'.

12. A high frequency tube structure comprisinga resonator body provided with a resonator cavity having substantially the shape of a frustum of a cone, said resonator having a reentrant wall portion also of substantially frusto conical shape; the inner end of said reentrant wallportion having an inner end-apertured for theiingross of electrons into said resonator andbeing made of a high nickel content alloy having a low thermal coefiicient of expansion, said resonator having an apertured wall portion opposite said reentrant portion, both of said'apertures being aligned and forming an electron permeable gap, a flexible diaphragm adjacent said reentrant portion, tuning means for varying th axial extent of said gap, said tuning means consisting in part of flanges connected to said reentrant portion and said apertured wall portion respectively, and a thermionic cathode structure contained at least partially within said reentrant portion for supplying electrons for passage through said gap.

13. High frequency tube apparatus comprising a cavity resonator having a reentrant portion, said reentrant portion being formed as a part of a tube whose external end is flared in a plane substantially perpendicular to the axis of said tube, a vitreous press butted to said flared portion, and a thermionic cathode supported by said press and extending within said reentrant portion.

14. A high frequency tube structure comprising a cavity resonator having a copper outer wall and an inner reentrant portion, said resonator havinga pair of aligned apertures defining an electron permeable gap, and an indirectly heated cathode located at least partly within said reentrant portion, the material of said reentrant portion having a low linear coefficient of thermal expansion, whereby variations in the axial spacing of said gap due to temperature-responsive changes in the linear extent of said reentrant portion resulting partially from the heat supplied to said cathode is minimized.

15. Apparatus as in claim 14 wherein the material of said reentrant portion is composed of a high nickel content alloy, said coefficient of thermal expansion being of the order of magnitude suitable for butt-sealing to glass.

16. High frequency tube apparatus comprising a cavity resonator having a first apertured end wall supporting a first grid, a centrally apertured flexible diaphragm providing a second end wall, and a reentrant portion including a cylindrical member extending through the aperture of said flexible diaphragm in the direction of said first wall and connected to said diaphragm, said reentrant portion having a second grid at one end thereof coaxial with and adjacent said first grid; a cathode coaxial with and located adja- 10 cent said second grid with at least a portion thereof contained within said reentrant portion; means for projecting an electron beam through said grids in a first traverse of said gap; means including a reflecting electrode positioned close behind said first grid for redirecting said electrons in a second traverse of said gap, said refleeting electrode being disc-shaped and having a flange at the periphery thereof axially extending in the direction of said cathode, whereby a close spacing of said cathode, said grids and said reflecting electrode is provided.

1'7.,iIn high frequency tube apparatus, the combination comprising a cavity resonator having conductive internal surfaces and adapted to contain a cyclically-varying electromagnetic field, said resonator further having a reentrant portion extending axially of said resonator, said portion having a first grid located atone end thereof and being connected to a flexible diaphragm,

said resonator also having asecond apertured end wall having a second grid contained at least partiallywithin said, aperture, said grids being closely spaced fromone another and defining an electron-permeable gap, a cathode, located adjacent said first grid and'at least partlycontain'ed within said reentrant portion, and'a disc sha'ped reflecting electrode positioned substantially close to said second grid and having a flange portion at the periphery thereof axially extending in the direction of said cathode, whereby close spacing of said first and second grids, said cathode and said reflecting electrode is provided.

18. A high frequency tube comprising an electron-emissive cathode electrode, means adjacent thereto for accelerating a beam of electrons from said cathode, a pair of control grids mounted in the path of said beam, and an additional electrode, said pair of control grids being adapted to be connected to a resonant circuit to impart velocity modulation to the electrons in said beam, one of said grids being spaced from the other of said grids by a substantial length of a metallic supporting member, said supporting member being made of a material having a very low thermal coefficient of expansion.

19. Apparatus as in claim 18 wherein the material of said supporting member is constituted of a high nickel content alloy.

20. A high frequency tube comprising an electron-emissive cathode, electrode means adjacent thereto for accelerating a beam of electrons from said cathode, a pair of control grids mounted in the path of said beam, and an additional electrode, said pair of control grids being adapted to be connected to a resonant circuit to impart velocity modulation to the electrons in said beam, one of said grids being spaced from the other of said grids by a substantial length of a metallic supporting member, said supporting member being made of a high nickel content alloy.

21. Apparatus as in claim 20 wherein the linear coefficient of thermal expansion of said supporting member is low, said coeificient of expansion being suitable for butt-sealing to glass.

22. An electronic discharge tube comprising an electron-emissive cathode electrode, and a pair of electrodes mounted adjacent thereto in the path of the electrons emitted from said cathode, one of said electrodes being spaced from the other of said electrodes by a substantial length of a metallic-supporting member, the operation of said tube being critically dependent on the spacing between said electrodes, said supporting member being made of a material having a very low thermal coefficient of expansion.

, 23. Apparatus as in claim 22 wherein said supporting member is composed of a high nickel content alloy, said coefficient of expansion being of the order of magnitude suitable for sealing to glass.

24'. Ahigh frequency electron discharge device comprising an electron emissive surface, cavity resonator means having longitudinally spaced apertures, said apertures being substantially aligned with said surface to permit the passage of'electrons therethrough, means for maintaining in the presence of temperature change the longitudinalspacing of said apertures substantially constant, said last-named means including a portion of said resonator means composed of a high nickel content alloy having a low thermal coefficient of expansion, said thermal coefficient being of the order of magnitude suitable for butt-sealing to glass.

25, A high frequency electron discharge device eomprising an electron emitter, a cavity resonator having a reentrant' portion with a first aperture at the end thereof, said resonator further having a second aperture registering with said first aperture, both of said apertures sIGURD' F. VARIAN.

REFERENCES CITED 2 The following references are of record in the I file of this patent: v

I UNITED STATES PA' I'EN'ISv Number Name ,QDate 2,190,511 Cage f Feb. 13, 1940 2,259,690 Hansen et a1. Oct. 21, 1.941 2,372,213; Litton Mar. 2'7, 1945 2,374,810 Fremlin May 1, 1945 2,406,850 Pierce Sept. 3, 1946 2,414,496 Varianetal. Jan. 2l, 1947 2,425,748 -Llewellyn-e Aug. 19,1947

FOREIGN P T NTS Number Country Date,

Great Britain Jan. 21, 1935 

